Streptococcus mutans is considered the principal etiological agent of human dental caries. The ability of the organism to survive a variety of harmful or stressful conditions and to emerge as numerically significant member of stable oral biofilm communities are essential elements for the persistence and cariogenicity of S. mutans. Such abilities are critical to understand how this organism colonizes and persists in specific niches, and are likely to interconnect with sophisticated adaptation mechanisms that allow the organism to maintain homeostasis within the dynamic oral microflora. This important aspect of S. mutans physiology will be pursued in this proposal by studying the homologous cidAB and lrgAB operons, recently identified as being highly balanced and coordinated during S. mutans growth in oxygen, as well as shown to play a significant role in a variety of key S. mutans virulence traits, including autolysis, biofilm formation, oxidative and heat stress, and genetic competence. More importantly, the cid and lrg operons were predicted to be analogous to bacteriophage-encoded holin/antiholin proteins (suggested to control the activity of bacteriophage-mediated cell death and lysis). The goals of this proposal are 1) to decipher the reciprocal regulatory circuits of regulatory feedback between the cid and lrg genes, 2) to identify and characterize the Cid/Lrg-mediated cellular and molecular mechanisms in S. mutans, and 3) to evaluate contribution of Cid and Lrg to the competitive fitness of S. mutans in-vitro and in-vivo. These objectives will advance our understanding of how S. mutans persists in biofilms that harbor a highly complex population of differentiated cells and dynamic metabolic processes, and represent a new avenue of bacterial programmed cell death research that will further fundamental knowledge relevant to other oral streptococci and Gram-positive organisms. Furthermore, establishing a link between biofilm development and cell death/lysis will provide another valuable insight into inhibition of the initiation or progression of dental caries and possibilities for improved therapy and disease control.